1. Field of the Invention
The present invention is broadly concerned with an improved method for removing cyanide ion from aqueous dispersions. More particularly, the preferred method is especially directed to cyanide ion removal from dispersions or solutions produced by the gold mining and electroplating industries, and involves reacting such dispersions at a very low pH with Fe+2 ion so as to complex the cyanide ion and permit easy separation thereof by filtration or other conventional techniques.
2. Description of the Prior Art
Cyanide is used as a reaction in the plating, gold mining and chemical industries and is also generated in the coke-making process. The cyanide from all of these sources eventually becomes an aqueous waste product. However, cyanide is extremely toxic and must be essentially removed from waste water before the water can be legally discharged. For example, in cadmium plating operations, the plating solution contains substantial quantities of sodium cyanide. After parts are cadmium plated in such a solution, they are rinsed in water. This rinse water inevitably becomes contaminated with cyanide ion at various levels, ranging from a few ppm up to a million ppm or more. Likewise, modern large scale gold mining operations now employ the cyanide heap leeching process for gold extraction. This process allows economical recovery of gold from relatively low grade ores. The process involves the oxidative dissolution of native metal in the presence of aqueous cyanide to produce aurocyanate, which is then treated to recover the gold. The cyanide leech process thus produces large quantities of cyanide-containing waste streams.
Various proposals have been advanced in the past for removing cyanide ion from contaminated waste waters. Busch et al., Ion-Precipitate Flotation of Iron-Cyanide Complexes, J. WPCF, 52(12):2925-2930 (1980) describe a process for removing ferricyanide using an ion-precipitation process with surfactant. These authors report that pH has very little effect on the recovery of ferricyanide. Hammen et al., Separation and Removal of Metal Cyanides, Proceedings of Hazmacon ""95, Hazardous Materials Management Conference (1995) describe a cyanide removal process using a modified porous silica anion-exchange media. U.S. Pat. No. 5,106,508 describes a process for heavy metal and cyanide removal from plating process waste streams wherein cyanide is oxidized by hypochlorite at pH 11.5.
These prior art approaches to the problem of cyanide removal suffer from problems of complexity and cost. This is especially significant when it is considered that relatively large volumes of waste waters must be treated having varying cyanide ion concentrations.
The present invention overcomes the problems outlined above and provides an improved method for removing cyanide ion from aqueous dispersion or solutions. Broadly speaking, the method involves reacting a cyanide ion-containing aqueous dispersion with Fe+2 ion at a very low peak pH of up to about 2 in order to form an iron-cyanide ion reaction product. This product is then conventionally separated from the dispersion by filtration, centrifugation or decantation.
In typical operations, the aqueous dispersion is a solution such as that derived from plating rinse waters. In such a case, the first steps is to normally lower the pH to a level of up to about 2 by the addition of acid (e.g., HCl), followed by the addition of an Fe+2 ion source (e.g., ferrous ammonium sulfate). This results in the formation of a deep blue reaction product Fe4[Fe(CN)6]3 which is relatively insoluble. The reaction product can then be separated.